1. Pteroylmonoglutamic acid (PteGlu) absorption has been measured by using the technique of small-intestinal perfusion with tritiated PteGlu in normal subjects and in patients with coeliac disease. 2. At similar intrajejunal pH, patients with untreated coeliac disease have significantly less PTEGlu than normal subjects and patients with treated coeliac disease. 3. The "resting" pH in the jejunum did not differ markedly between normal subjects and patients with coeliac disease. 4. Increasing pH decreased PteGlu absorption in patients with coeliac disease and in normal subjects. 5. These findings suggest that PteGlu malabsorption in coeliac disease is not due to abnormally high pH in the jejunum.